An electric discharge machine ("EDM"), such as that described in Japanese Patent No. 58-40225, has a transistor switch placed between the gap and the power source. The transistor switch is periodically turned on and off for short predetermined periods which defines a machining cycle. Within the "on" time of a machining cycle, the transistor may be intermittently triggered on and off. The intermittent turning on and off of the switching transistor maintains a generally continuous machining current.
When conditions are such that the machining current should be lowered during the predetermined "on" time, the current is cutoff. Therefore, the actual "on" time during a machining cycle is less than the predetermined value of "on" time. This premature cutting-off of the machining current results in reduced machining speed and in faster wear down of the electrodes.
Japanese Patent No. 62-5733, discloses a circuit where the machining current cutoff can be eliminated by the insertion of an inductor in series with the gap. Although this inductor reduces the machining current cutoff, it also increases the time constant of the circuit. The increased time constant causes the rise time of the machining current to increase. This in turn causes a drastic reduction in the machining speed, which is the exact opposite of the desired effect.